Arthropod Diet of Pileated Woodpeckers in Northeastern Oregon

Abstract

We examined 330 samples of scat collected from 23 individual pileated woodpeckers (Dryocopus pileatus) from August 1989 to August 1990. Based on arthropod mandibles found in the scats, the diet was 68% carpenter ants (Camponotus spp.), 29% thatching ants (Formica spp.) and other ants, 0.4% beetles (Coleoptera), and 2% other. Diet changed seasonally with none or few (0-11%) of the thatching and other ants found from November to February, but these ants comprised most of the diet from June until September. Carpenter ants were found in the diet in all months. Scats collected from the roost cavity of one bird contained 170 mandibles of western spruce budworm (Choristoneura occidentalis) and 259 mandibles of an unknown Isoptera. Knowing the diet of any species is important in understanding its foraging ecology. Diet of the pileated woodpecker (Dryocopus pileatus) has been described in the eastern United States and includes insects (primarily ants and wood-boring beetle larvae) all year and some fruits and nuts in the fall (Conway 1957; Hoyt 1957). Bent (1939) reported stomach contents of three birds to contain ants, beetle larvae, and fruits and seeds. Beckwith and Bull (1985) found that this species feeds primarily on ants in northeastern Oregon. Our objective was to determine the arthropod diet of D. pileatus over time in northeastern Oregon. STUDY AREA AND METHODS The study was conducted in the Blue Mountains in northeastern Oregon in Baker, Umatilla, Union, and Wallowa Counties. We collected scats from birds in six study areas within 200 km of either La Grande or Wallowa, Oregon. The study areas were in mixed conifer forests; scattered grasslands comprised 4-14% of these areas. The forests were predominantly mature, uneven-aged stands of grand fir (Abies grandis), Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), western larch (Larix occidentalis), and lodgepole pine (Pinus contorta). Two of the study areas had been high-graded 40 years ago, three had partial overstory removals about 20 years ago, and one had been extensively logged (clearcut and shelterwood) within the last 15 years. We captured 23 adult D. pileatus in summer (mid-June to mid-August) 1989 and attached 11 g 2-stage transmitters to them. Each bird was followed 1-3 times each week for 1-3 hours until March 1990 when we retrapped the birds and removed the transmitters. During the time each bird was followed, we collected scats when possible. To locate fresh scats, we searched each foraging site or resting site after the bird flew away. We found scats in June 1989, in each month from August 1989 to March 1990, and in August 1990. The scats found 1 August 1990 were collected from the roost cavity of a pileated woodpecker. These probably had been defecated in July because the bird had been roosting in this cavity for several weeks, so we included this sample in July. Scats were difficult to collect in June and July because the birds were often feeding in the tops of trees, and scats were virtually impossible to find on the ground. The characteristic large size of the scat is unique to this woodpecker, and we used the technique of scat analysis described by Beckwith and Bull (1985). This technique primarily identifies arthropod parts because plant material may not be detected. We assumed that the mandibles and other arthropod remains in the scat represented the entire diet. We collected 330 samples: 313 were subsampled, 11 were used in a regression analysis, and 6 (32 individual scats) were collected from a roost. Each of 313 scat samples was dried for 12 hours at 300C and weighed to the nearest 0.001 g on an analytic balance. A subsample was taken for examination